The present invention relates to a high-gain transistor amplifier, and more particularly, to a transistor amplifier suited for a semiconductor integrated circuit in which the amplified output is free from noise carried on the power voltage source.
A conventional high-gain transistor amplifier is constructed with at least two cascade-connected transistors. In more detail, an input signal is applied to the base of a first transistor having a collector connected directly to the base of a second transistor and to the power source through a first load resistor. The output is derived from a second load resistor connected to the collector of the second transistor.
In general, the power source produces a DC power voltage from power taken from the commercial electric mains. It is difficult to completely suppress the hum, ripple and other noise present thereon in the output DC power voltage. The hum, ripple and other noise are applied from the power source to the second transistor in the amplifier circuit through the first load resistor and amplified by the second transistor. As a result, amplified hum, ripple and other noise are present on the output of the amplifier, thus producing a low S/N ratio.
For improving the S/N ratio, it has been proposed to divide the first load resistor into first and second parts and to ground the connecting point of the divided load resistor through a large value capacitor. This improvement is effective to improve the S/N ratio. It, however, is accompanied by a difficulty in circuit design.
More specifically, as the load impedance of the first transistor equals the first part of the first load resistor, namely that part of the first load resistor connected between the collector of the first transistor and the large capacitor, the collector current of the first transistor is determined by the total resistance of the divided load resistors. Therefore, the load impedance cannot be made larger than the total resistance required for the collector current of the first transistor. Further, the hum, ripple and other noise cannot be prevented from being applied to the amplifier unless the time constant of the circuit composed of the capacitor and the second part of the first load resistor, namely that part of the first load resistor positioned between the capacitor and the power supply, is made large. However, if the resistance of the second part of the first load resistor is made large, there arises another problem that, since the first part of the first load resistor cannot be made large, the load resistance of the first transistor is compelled to be small to sustain the desired magnitude of collector current. This results in a small voltage gain of the first transistor.
Thus, it is difficult to design the circuit to meet all the requirements regarding the collector current and the load resistance of the first transistor and the time constant for suppressing hum, ripple and other noise.
Recently, transistor amplifiers have been implemented in the form of semiconductor integrated circuits for purposes of cost and size reduction. A prior art amplifier as described above meets another difficulty when it is formed as a semiconductor integrated circuit, specifically, the large value capacitor added in the improved amplifier, which is very expensive in any event, cannot be formed on a semiconductor chip. Consequently, besides those to which the other circuit elements are connected, external terminals (bonding pads) for the capacitor must be provided on the semiconductor chip, thereby resulting in an enlargement of the semiconductor chip, an increased cost, and a low production yield.
Referring to FIG. 1 illustrating a circuit diagram of a prior art high-gain transistor amplifier, an input signal 1 is applied to the base of a transistor 4 via a capacitor 2. The transistor 4, which operates as a common emitter amplifier, has its collector connected to a power source 8 through a load resistor 5 and a filter including a resistor 7 and a capacitor 6. The voltage produced across the load resistor 5 is applied to the base of a transistor 11. The transistor 11, the collector of which is connected to a power source 8 through a load resistor 9, provides an amplified output to an output terminal 10. To operate the transistor 11 in a common emitter mode, the emitter of the transistor 11 is grounded through a capacitor 13 in an AC mode but through a resistor 12 in a DC mode.
The DC voltage produced at the emitter of the transistor 11 is used as a base bias for the transistor 4, being connected thereto through the resistor 3.
This circuit may be formed as a semiconductor integrated circuit by forming transistors 4 and 11, resistors 5, 7, 9 and 12 on a single semiconductor chip. Such an integrated circuit is provided with terminals 10, 24, 25, 26 and 27 for connecting to the chip the external circuit elements including the capacitors 2, 6, 13 and the resistor 3, the power source 8 and input and output circuits.
In this circuit, since the circuit has a negative feedback path for the DC mode, the approximate DC bias voltage at the emitter of the transistor 4 is 0.7 volts, which is equal to the forward-biased base-emitter voltage of the transistor 4. The emitter voltage of the transistor 11 is 0.7 volts since it is substantially equal to the base voltage of the transistor 4. The base voltage of the transistor 11 is 1.4 volts, which is 0.7 volts higher than that of the emitter.
If the circuit were not provided with the capacitor 6, when noise such as hum or ripple is present on the output voltage from the power source 8, the noise would pass through the resistors 7 and 5 and would be amplified by the transistor 11 with the amplified output appearing at the output terminal 10. As a result, a large amount of noise would appear at the output terminal 10. The capacitor 6 is inserted in order to alleviate this problem. By grounding the connection point of the resistors 5 and 7 through the capacitor 6, the noise component from the power source 8 is considerably attenuated at the collector of the transistor 4.
This measure is effective for preventing the noise carried on the power source voltage from reaching the output terminal. It, however, has many problems as mentioned hereinunder.
The collector current of the transistor 4 is given approximately by ##EQU1## where V8 is the voltage of the power source 8 and R7 and R5 are the resistance values of the resistors 7 and 5. Thus, the collector current of the transistor 4 is determined by the total resistance value of the resistors 5 and 7. On the other hand, the load impedance is determined by the resistance value of the resistor 5 and cannot be made larger than the total resistance value of the resistance 5 and 7. Further, the resistor 7 is required to be large in order to sufficiently reduce the noise component carried on the power source voltage. This is because the amount of reduction of the noise depends on the time constant of the filter formed by the resistor 7 and the capacitor 5.
It is difficult to design the circuit to meet all the requirements relating to the collector current, the load impedance and the time constant of the filter. This difficulty is even more serious in the case where the amplifier is powered with a low power source voltage.
Further, the value of the capacitor 6 is generally chosen to be 10 to 500 .mu.F so that the noise component will be considerably reduced. Such a large capacitor cannot be formed on a semiconductor chip. Thus, if the amplifier is formed as a semiconductor integrated circuit, the capacitor 6 must be externally connected to the integrated circuit. Thus, the integrated circuit requires at least one additional terminal. The provision of this additional terminal lowers the integration density and the production yield.